Single crystals are used in the manufacture of a wide variety of products, such as integrated circuits, optical systems and various other devices. The Bridgman-type methods are often used to grow single crystals from a melt. A synopsis of these methods is given below.
In the vertical Bridgman method the polycrystalline charge is melted in a tube-like container positioned in the hotter portion of a vertical furnace. The container is then lowered slowly into the cooler portion of the furnace to cause the melt to solidify from the bottom of the container. Alternatively, the container can remain stationary while the furnace is raised slowly. The container is designed such that its bottom is reduced gradually to a tube smaller in cross-section to help the melt solidify into a single crystal. If desired, a seed crystal can be placed in the bottom tube.
In the horizontal Bridgman method the polycrystalline charge is melted in a boat-like container positioned in the hotter portion of a horizontal furnace. The container is then pulled slowly into the cooler portion of the furnace to cause the melt to solidify. Alternatively, the container can remain stationary while the furnace is pulled in the opposite direction. The container is designed such that the end at which solidification begins is reduced gradually to a smaller cross-section to help the melt solidify into a single crystal.
The gradient-freeze method is in fact a Bridgman-type method. The container for crystal growth is identical to that in the Bridgman method. The charge is also melted in a container which is positioned in the hotter portion of the furnace. However, the container and the furnace both remain stationary. The location of the steep temperature gradient between the hotter and cooler portions is then caused to shift along the furnace. This in turn causes the melt to solidify along the container like in the Bridgman method. Crystals can be grown either vertically or horizontally.
Single crystals are often alloyed or doped with a solute to develop the desired physical properties. The solute can be an element or a compound, and its concentration can range from as high as a few per cent or greater to as low as around 10.sup.17 .about.10.sup.18 atoms per cubic centimeter.
At the growth front, i.e., the interface between the crystal and the melt, the solubility of the solute in the crystal C.sub.S often differs significantly from that in the melt C.sub.L. This difference is measured by a segregation coefficient defined as k=C.sub.S /C.sub.L. For k&lt;1, the solid cannot hold as much solute as the melt. As such, the solute is rejected by the growing crystal into the melt. This causes the melt solute concentration at the growth front C.sub.L to increase, which in turn causes the crystal solute concentration at the growth front C.sub.S (=kC.sub.L) to increase. Consequently, the solute concentration increases along the axis of the resultant crystal. For k&gt;1, the opposite is true. In either case the extent of solute segregation increases with the extent of convection in the melt.
Most attempts to control solute segregation in Bridgman-type crystal growth can be grouped into two categories, i.e., melt convection control and melt composition control. In the first category convection in the melt is reduced. A magnetic field was used to damp convection. See, e.g., H. P. Utech et al., J. Applied Physics, vol. 37, 1966, p. 2021. Microgravity was used to reduce buoyancy convection. See, e.g., A. F. Witt et al., J. Electrochemi. Soc., vol. 122, 1975, p. 276. Centrifugation was also used. See H. Rodot et al., J. Crystal Growth, vol. 104, 1990, p. 280. A disk heater was submerged in the melt to suppress convection near the growth front. See A. G. Ostrogorsky, U.S. Pat. No. 5,047,113. A series of closely spaced partitions were immersed in the melt to reduce convection. See Japanese Pat No. 62,012,691. These methods are not effective for k far from unity.
In the second category, which is not subject to this deficiency, variations in the composition of the (first) melt are compensated by replenishing with a material of a different composition and segregation is thus reduced. A solid was dropped into the melt in vertical Bridgman crystal growth. See Japanese Pat. No. 61,077,694. A second melt was provided in a crucible above the first melt in vertical Bridgman crystal growth and allowed to drip by itself from a bottom hole of the crucible into the first melt. See Japanese Pat. No. 62,148,390. A second melt was provided in a crucible immersed in the first melt in vertical gradient-freeze crystal growth and allowed to leak from a bottom hole (1 mm long and 1.5 mm in diameter) of the crucible into the melt. See Japanese Pat. No. 02,167,883. In vertical Bridgman crystal growth with a submerged disk heater, a second melt was provided over the heater to feed the first melt under it. See A. G. Ostrogorsky et al., J. Crystal Growth, vol. 128, 1993, p. 201. A second melt was enclosed in a cylinder-piston assembly immersed in the first melt in horizontal Bridgman crystal growth and injected into the first melt through the bottom hole (around 1 mm long) of the cylinder. See Japanese Pat. No. 61,247,681.
Dropping solid into the melt can result in polycrystals if the solid is not melted completely. The feed rate is not controlled if the second melt drips by itself from a crucible. Variations in the feed rate can cause the composition of the first melt to vary, which in turn causes the composition of the growing crystal to vary. In fact, dripping from the bottom hole of a crucible can be erratic if a melt does not wet the crucible, e.g., a semiconductor or metal melt in a quartz or boron nitride crucible. A short passageway connecting a second melt to the first melt can be insufficient for suppressing solutle diffusion between the two melts. A hole through the thin bottom wall of an immersed crucible or cylinder is often too short. In some cases even the annular space between a submerged disk heater and the inner wall of the crystal growth container can be too short. Diffusion causes the compositions of both melts to vary, which in turn causes the composition of the growing crystal to vary.